Snapper Arthur Edwards, who has been photographing the royals for decades, said duty and service “mean everything” to the monarch, 95. He added that she is “determined” to carry on for as long as possible.
Mr Edwards also said the Queen’s aides will struggle to persuade her to slow down.
The photographer said: “The problem her aides will now face is trying to stop her doing too much.
“Her pledge to serve the nation at her coronation in 1953 is as firm now as it was back then.
“This is a woman for whom duty and service mean everything.
“Her role is her life.
“And she is determined to carry on that role as long as she can.”
Mr Edwards insisted that the Queen will never abdicate, but may pass more duties to the Prince of Wales and the Duke of Cambridge.
Writing for The Sun, he said: “So, don’t expect an abdication.
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The monarch was said to be back at her desk and carrying out light duties on Thursday afternoon following her return to Windsor Castle.
The Queen has had a gruelling schedule of royal engagements in recent weeks.
She hosted a major Global Investment summit at Windsor Castle on Tuesday evening.
The head of state, who is just five years away from her 100th birthday, is due to celebrate her Platinum Jubilee – 70 years on the throne – next year.
She still keeps a busy diary of events and audiences and deals with her daily red boxes of official papers.
On Tuesday, it was revealed the Queen had politely turned down the Oldie of the Year trophy because she believes “you are only as old as you feel”.
The monarch, who cancelled a trip to Northern Ireland this week, is due to host a reception in Scotland on November 1 for the Cop26 climate change conference.